1. Field of the Invention
The present invention relates to a brazing alloy for joining a composite assembly made up of a pair of members at least one of which is a metallic member or one of which is a metallic member and the other is a ceramic member. Further, the present invention relates to a composite assembly such as a tappet for an internal combustion engine, which is joined by using a brazing alloy.
2. Description of the Prior Art
An active brazing alloy has heretofore been used for joining a ceramic member and a metallic member of a composite assembly. An example of such a composite assembly having been practically used, is a turbocharger rotor, ceramic tappet, etc. The active brazing alloy contains active metals such as Ti, Zr, etc. The active brazing alloy is used exclusively with a view to increasing the strength at the joint between the ceramic member and the metallic member. By the use of such an active brazing alloy, a reaction layer is formed between the brazing alloy and the ceramic member to firmly bond the same and thereby enable the ceramic member and the metallic member to be joined firmly.
Further, among active brazing alloys, low melting point brazing alloys (solidus point is from 600.degree. C. to 750.degree. C.) such as In--Cu--Ag--Ti brazing alloy are used for joining the members of a mechanical part assembly such as a sliding part for an automobile which is used at a low temperature, with a view to reducing the difference in shrinkage (thermal distortion) between the ceramic and metallic members due to the difference in thermal expansion coefficient therebetween. It has been developed a technique for directly brazing a Si.sub.3 N.sub.4 ceramic member and a Ni--Cr--Mo (SNCM 630) metallic member with an In--Cu--Ag--Ti brazing alloy for thereby manufacturing a tappet, rocker arm, valve bridge, etc. as disclosed in Japanese Patent Provisional Publication No. 2-199073.
Further, high melting point brazing alloys (solidus point is 900.degree. C. or higher) of active brazing alloys have also been practically used.
The composite assembly joined with a low melting point alloy of the above described prior art active brazing alloys, has a sufficient strength with respect to tensile stress, compression stress, torsional stress, etc. at the initial stage of usage.
However, when used as an actual article, the composite assembly joined with such a low melting brazing alloy cannot produce a sufficient strength since it is subjected to severe conditions such as thermal stress other than the above described simple stress due to the difference in thermal expansion between the members caused by variations of temperature and fatigue caused in an elongated period of usage.
For example, in the case of a tappet for an internal combustion engine, which has a ceramic member formed with a sliding surface for contact with a cam, the temperature at the joint between the ceramic member and the metallic member (engine tappet main body) increases up to about 200.degree. C. due to the heat caused by the slide of the cam and the tappet relative to each other, the heating by the engine lube oil, etc. Such a temperature increase causes thermal stress due to the difference in thermal expansion between the ceramic member (e.g., Si.sub.3 N.sub.4) and the metallic member. Further, the fatigue is caused in an elongated period of usage. As a result, in use of such a tappet, there may occur such a case in which deterioration in the joining of the ceramic member and the metallic member such as separation or detachment thereof is caused. In this connection, the strength of joining between the metallic member and the ceramic member depends on the wettability of the joining surface of the metallic member. However, there is a limit in improving the wettability and preventing the detachment or separation of the metallic member from the brazing alloy layer.
Further, by the brazing with a high melting point active brazing alloy, it may be possible to prevent the strength of the brazing alloy of itself from being lowered in response to increase of the temperature. However, it has been impossible to prevent separation of the metallic member from the brazing alloy layer due to the stress caused by the difference in thermal expansion between the metallic member and the ceramic member in use.